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On May 2, 2011, about 1440 Pacific daylight time, a Cessna 421B, N270CS, experienced an uncommanded retraction of the right flap during the downwind leg for landing at Truckee-Tahoe Airport, Truckee, California. California Shock Trauma Air Rescue (CALSTAR) was operating the airplane under the provisions of Title 14 Code of Federal Regulations Part 91 as a repositioning flight. The commercial pilot and two medical flight crewmembers were uninjured, and the airplane was undamaged. The flight departed McClellan Airfield, Sacramento, California, at 1412, with a planned destination of Truckee. Visual meteorological conditions prevailed, and an instrument flight rules flight plan had been filed.
The pilot reported entering the left downwind leg for runway 28 about 8,500 feet mean sea level (msl). He stated that the pattern altitude was 7,500 feet, so in an effort to descend, he reduced airspeed to 140 knots, extended the landing gear, and incrementally deployed the flaps to 45 degrees. About the time the flaps reached their maximum limit, he heard a popping sound, and the airplane simultaneously rolled about 80 degrees to the right. He countered with full left aileron control input, retracted the flaps, and applied full engine power. Once he had attained a positive rate of climb, he retracted the landing gear.
The pilot stated that he was only able to maintain wings-level with full left aileron control input, and full left roll trim, and as such, assumed the airplane had experienced a failure of the right flap. He circled Truckee airport to troubleshoot further, but was unable to extend the right flap, or retract the left. He checked the flap motor circuit breaker, and it was in the closed position. The flap indicator needle was in the zero flap position, and the flap drive motor would not engage, regardless of the flap handle position.
He assessed the possibility of landing at Truckee, Reno, or South Lake Tahoe airports, but due to terrain and wind concerns, he elected to divert back to McClellan. He was able to accomplish left turns with about 5 degrees of bank, and although right turns could be performed, recovery to wings level was slower than normal. In this configuration he was able to maintain a climb rate of about 300 feet per minute, and once the airplane reached 13,000 feet msl, he was able to maintain wings-level by slightly relaxing the control input to about 90 percent deflection.
For the remaining 35 minutes of flight, the pilot employed the assistance of a medical crewmember to help with maintaining left aileron control deflection. The pilot subsequently landed the airplane at McClellan without incident, after making a left downwind approach.
TESTS AND RESEARCH
Flap Control System
The flap control system for the 421B series airplane is operated by an electric motor, which drives a gear reduction unit. Two sprockets, connected in tandem to the reduction unit output shaft, drive four chain-connected cables, which actuate bellcranks in each wing. The flaps are then driven by push-pull tubes connected to the bellcranks. A cam, driven by a gear attached to the aft output shaft of the reduction unit, operates two limit switches. The flap preselect system is comprised of a preselect lever assembly, mounted on the instrument panel; a flap preselect control cable, attached to the preselect lever assembly and routed to the upper right wing flap extend cable; and associated microswitches and electrical wiring. When the preselect lever is moved to a flap position, the microswitches in the lever assembly are energized, and actuate the flap motor. As the flaps reach the preselected position, the preselect lever assembly microswitches are de-energized.
Initial Flap System Examination
During a cursory inspection of the flap system by the operator, it was discovered that the right wing upper flap extend cable, part number 5000008-62, had separated about 11 inches from the end of the inboard turnbuckle and chain fitting, inboard of the preselect cable clamp. Additionally, a steel braided oxygen system line had come loose, and had fallen into the path of the flap cable in the area of the failure. Based on this finding, and with the permission of the NTSB investigator-in-charge (IIC), the operator removed all the flap cables and their associated pulleys for examination. Examination of the cables and oxygen line revealed that they had not made contact, and in fact, the location of the separation was adjacent to the contact area of the inboard fuselage pulley. Additionally, the corresponding left wing lower flap extend cable, part number 5000008-63, exhibited frays to multiple strands in the area adjacent to its corresponding inboard fuselage pulley.
The airplane's flap system was subsequently examined by the IIC. The flap actuator motor assembly was free of damage. When the upper and lower limit switches were triggered by hand, they produced an audible click, and disengaged the motor appropriately when power was applied to the system.
The flap preselect cable remained intact, and was continuous from its connecting clamp through to the preselect assembly on the instrument panel. The preselect assembly appeared intact, with its limit switches and electrical wires firmly in place. The switch produced an audible click when engaged utilizing the flap lever.
The upper right, and lower left, flap extension cables, along with their associated inboard pulleys, were sent to the NTSB Materials Laboratory for examination.
Materials Laboratory Examination
Both cables were similar in construction, with 1/8-inch diameter 7X19 wire ropes with a turnbuckle fitting on one end, and swiveling clevis fittings on the other. Both cables were approximately 63 inches in length.
The right-hand extend cable had separated at a location approximately 11 inches from the end of the turnbuckle fitting. Examination revealed that most of the individual wire fractures were within a 0.5-inch-long portion of the cable, with the remaining wires separated 2 inches further away. High magnification imaging of individual wire fractures revealed fatigue cracking on more than half of the approximately 24 individual wires that were examined. The other wires displayed necking fracture features consistent with overstress separations.
No external wear was noted along the entire length of the cable, and diameter measurements did not indicate any significant internal wear. Fraying was detected in one region, about 20 to 25 inches from the clevis end, in a region adjacent to the outboard wing stub pulley. Magnified examinations revealed at least 24 individual broken wires within that region.
The left-hand extend cable was visually examined, and about 20 individual wire fractures were present and concentrated in an area between 51 and 53 inches from the outboard clevis end.
Both inboard pulleys were free of obvious wear, and turned smoothly on their bearings. Refer to the Materials Laboratory Report within the docket for specific examination details.
The airplane had accrued 4,832.1 total flight hours at the time of its last annual inspection, on March 19, 2011. Examination of the maintenance records revealed that the flap extend cables had not been replaced since the airplane's manufacture in 1975. No life limits exist for the flap cable.
The Cessna 421B service manual does not require the removal of flight control cables during inspection. Rather, the inspection calls for visual examination along the entire cable length, and physical examination with a cloth over cables at pulleys, fairleads, pressure seals, and other areas the cable may be subject to chafing or wear. The manual states that individual broken wires are acceptable, provided no more than three individual wires are broken in any given 10-inch cable length.
The mechanic who performed the most recent inspection reported that he examined the flap cables utilizing the methods prescribed in the Cessna service manual, but did not detect any damage. He further stated that the frays in the cables were only noticeable after they were removed, and then flexed and looped by hand.
Service Difficulty Reports
A review of FAA Service Difficulty Reports (SDR's) for the Cessna 401 through 425 series airplane revealed 33 instances of flap cable wear or failure on 25 separate airplanes. Fourteen of the SDR's documented cable failures, all of which occurred during the landing approach phase, and resulted in an asymmetric flap condition. Some of the reports documented that the airplanes entered unusual attitudes just after the cables failed. The majority of these reports did not specify whether the flaps could be retracted, however, four reports specifically stated that the extended flap could not be retracted, and that the pilot landed the airplane with an asymmetric flap condition. In these instances, the reports sited various reasons for the flap not retracting. These included damage to the gearbox assembly caused by the forced retraction of the flap, the failed cables becoming entangled in the retraction mechanism, or the preselect cable being driven to the zero flaps position, thereby interfering with the electrical logic of the flap system.
None of the reports documented a separation of any left flap cables in flight, rather the separated cables were either the right extend type, or not specified. The separations all occurred in the area of the inboard fuselage pulley and preselect cable clamp.
Of the reports that did not describe an asymmetric flap condition in flight, four stated that frayed flap cables were observed during routine inspections. Fourteen of the remaining reports indicated that frayed cables were observed either during rigging procedures, or after the flap cables were removed, and that the position and nature of the damage prevented detection during routine inspections.
The total flight time for each of the damaged cables varied between 3,049 and 13,636 hours.